The operators of communication networks such as the conventional telephone network often provide the subscribers or customers not only with useful data transmission (speech in the telephone network) but also with a large number of other services or subscriber service features. These services include, for example, conference circuits, the transmission or suppression of telephone numbers, call forwarding or the charge pulse.
Inter alia, the characteristics of the subscriber line interface circuit (SLIC) and of the subscriber access line from the operator of the telephone network must be checked in the event of subscriber problems or else in the course of routine checking or monitoring of these guaranteed services. The subscriber access line connects at least one terminal of the corresponding subscriber to the associated subscriber line interface circuits in the switching center.
During a check such as this of the characteristics of the subscriber access line, connected terminals should, however, not respond, that is to say, for example, no call signal should be generated.
One of the tests during a check such as this is to investigate the behavior of the subscriber line interface circuit and of the subscriber access line connected to it when a positive supply voltage is connected.
Conventional test procedures associated with the prior art disadvantageously require an external test unit or measurement device for this purpose. This is connected to the appropriate subscriber line module analogue, and the positive ringing voltage is then measured by means of this measurement device. If the ringing voltage does not reach a specific threshold value, then this makes it possible to deduce that there is a fault on the positive supply voltage. In order to prevent the corresponding terminal of the contracted subscriber from reacting to the ringing voltage, the subscriber access line of the subscriber must additionally be decoupled from the switching center during the test process.